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T
he laser had its conceptual beginnings in the early 1950s, when many scientists
were engaged in radio frequency and microwave spectroscopy. This field
evolved vigorously after World War II, due in part to wartime breakthroughs in
radio frequency and microwave technology. These breakthroughs did much to tip the
scales of warfare in favor of the Western Allies, principally in their air and naval defense efforts, and contributed measurably to successes in air warfare over Great Britain
and antisubmarine warfare in the North Atlantic. The principal centers for radio and
microwave research were in Great Britain and the United States.
EARLY MICROWAVE SPECTROSCOPY
The first report of spectroscopy conducted at microwave frequencies was that of
Cleeton and Williams at the University of Michigan in 1934.1 The object of their study
was the ammonia gas molecule, NH3. Through spectroscopic studies, fine structure
had been observed in ammonia’s infrared rotational bands. This structure was interpreted as being due to the tunneling of the nitrogen atom through the plane of the
three hydrogen atoms (see Section 8.2). Theoretical studies of the ammonia molecule’s infrared spectrum had suggested that its doublet’s wavenumber separation would
be of the order of 1 wavenumber, cm1, which falls outside the infrared but within the
microwave region.2
To obtain a source of radiation of this energy, magnetrons were borrowed from the
Westinghouse Corporation’s research laboratory, but they generated radiation of much
lower frequencies than those required for the proposed work on ammonia. Using the
Westinghouse magnetrons as models, Cleeton and Williams constructed several magnetrons by scaling down the dimensions. To measure wavelength, they constructed a
device similar in operation to an echelon (a diffraction grating consisting of a series of
plates of equal thickness, stacked like a staircase). They succeeded in observing the inversion at a wavelength of approximately 1 centimeter, just where the ammonia infrared data had suggested.
After this early beginning, microwave spectroscopy remained dormant until the
end of World War II, largely because microwave technology had not yet developed
to the point where spectroscopy in the centimeter wavelength range could be performed. While working on a project for the war effort, some scientists at the MIT Radiation Laboratory observed an unexplained attenuation in test microwave signals
directed across the Charles River from Cambridge to Boston. The phenomenon
was most prominent at a time when a local garbage scow released waste products into
that river. Later, after the advent of microwave spectroscopy, the cause of this microwave signal attenuation was found to be the effluence of ammonia gas from waste
discharge!
Microwave spectroscopy is a consequence of transitions between energy levels,
which usually generate spectra in the microwave region. Radio frequency spectroscopy,
by contrast, is the result of the fact that nuclear electric quadrupole moment and
nuclear-magnetic dipole moment energy separations lie in the radio frequency region
of the electromagnetic spectrum. In the case of nuclear electric quadrupole moment
transitions, an interaction between nuclear electric quadrupole moments and local
electric field gradients causes the partial removal of nuclear-spin angular momentum
1C.
2D.
S. A. Marshall
Roosevelt University, Chicago, Ill.
E. Cleeton and N. H. Williams, Phys. Rev. 46:235, 1934.
M. Dennison and J. D. Hardy, Phys. Rev. 39:938, 1932.
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degeneracy. Nuclear magnetic resonance, on the other hand, is due to the removal of
nuclear-spin angular momentum degeneracy by an external magnetic field.
By the mid-1950s, the physical science and engineering journals were filled with research papers dealing with radio frequency and microwave spectroscopy. During this
decade, significant research in either of these fields might be conceived, executed, and
submitted for publication within a single month. Toward the end of the 1950s, microwave spectroscopy was headed for a crisis of sorts, because the supply of unstudied
and relatively uncomplicated gas molecules with spectra in the centimeter wavelength
region was being depleted. One way to open up new areas of investigation was to extend the wavelength range of microwave spectroscopy to the millimeter and submillimeter region, thereby including molecules of greater moment of inertia. However,
there seemed to be no way to obtain monochromatic radiation of constant amplitude
from vacuum tubes at wavelengths significantly below 3 mm.3
The development of coherent sources of radiation in the centimeter, millimeter,
and submillimeter wavelength region took place in two steps, both of which involved
the ammonia molecule. The first occurred in 1952 with a report by H. Lyons of the Central Radio Propagation Laboratory of the National Bureau of Standards in Washington,
D.C.4 Lyons described the performance of a clock — a quartz-crystal-controlled oscillator slaved to the peak absorption of the (3, 3) rotation inversion line of the ammonia
molecule. The notation (3, 3) signifies the J 3 and K 3 angular momentum states
of a symmetric top rotor, with J representing the angular-momentum quantum number
of the molecule’s rotation about its principal axis and K describing rotation about an
axis perpendicular to the principal axis.5 To first order of approximation, the ammonia
molecule does not exhibit an electric dipole moment because the nitrogen atom tunnels through the plane of the three hydrogen atoms, thereby causing its electric dipole
moment to reverse its direction relative to the molecule’s principal axis of symmetry
(see Section 7.2). In second order, however, its electric dipole moment is nonvanishing
so that, in fact, it exhibits a robust absorption at 23 870.127 MHz in the microwave
region. This clock became known as the ammonia clock. When compared with the
primary standard quartz-crystal-controlled oscillator clocks at the National Bureau of
Standards, the ammonia clock proved to have a stability of the order of a few parts in
109. The ammonia clock used the molecule’s rotation inversion absorption line as a
reference for its own quartz crystal-controlled oscillator, thereby providing an oscillator
of hitherto unprecedented stability.
The second step was a noteworthy advance in 1953, when J. Weber of the University
of Maryland provided, for the first time, a theoretical strategy for obtaining coherent
amplification of electromagnetic radiation by stimulated emission.6 The first step, he
said, is to create a thermodynamical nonequilibrium distribution of energy levels in a
crystalline or gaseous substance. Using the Einstein coefficients for spontaneous and
induced transitions,7 Weber demonstrated the necessary conditions for the coherent
amplification of radiation. Oscillations could be sustained if proper control of feedback and gain were provided to compensate for radiation losses. In his paper, Weber
estimated the performance of a gas at low pressure. As a choice of an active species, he
suggested the gaseous ammonia (3, 3) rotation inversion line, whose absorption is one
of the strongest in the microwave region. For a gas pressure of 102 torr and a
frequency of 30 GHz, he estimated that gains of the order of 0.02 dB per meter of
3W.
Gordy, Annals of the New York Academy of Sciences 55:774, 1952.
Lyons, Annals of the New York Academy of Sciences 55:831, 1952.
5C. H. Townes and A. L. Schawlow, Microwave Spectroscopy, New York, McGraw-Hill Book Co., 1955.
See p. 312.
4H.
6J.
Weber, Trans. IRE Prof. Grp. Electronic Devices, PGED-3, vol. 1, 1953.
Einstein, Mitt. Phys. Ges. Zurich 16(18):47, 1916; Z. Für Phys. 18:121, 1917.
7A.
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THE INVENTION OF THE LASER
ammonia could be achieved, resulting in power levels of 2 mW per 100 cm 3 of ammonia. He pointed out that amplification would die out within a period of the order of
the molecule’s relaxation time unless the system were maintained in a nonequilibrium
thermodynamic condition.
THE FIRST MASERS
Within a year or so of Weber’s publication, two molecular devices for generating microwave radiation were reported by N. G. Basov and A. M. Prokhorov of the Lebedev
Institute in Moscow8 and by J. P. Gordon, H. J. Zeiger, and C. H. Townes of Columbia
University in New York.9 Both devices used the ammonia molecule as the active species.
The acronym maser was coined to stand for “microwave amplification by stimulated
emission of radiation.” In particular, the Columbia University maser made use of molecular beam technology.
As described in Section 7.2, the ammonia molecule can be represented by one of
two configurations (see Fig. 7.14). In one (the right-handed molecule), the nitrogen
lies above the plane of the hydrogen atoms. In the other (the left-handed molecule),
nitrogen lies below the plane of hydrogen atoms. Note that no rotation can make
these two molecules identical, since inversion through center-of-mass coordinates
is not a proper rotation. Both right-handed and left-handed descriptions are valid
for the ammonia molecule, so an acceptable description of the ground state of ammonia must be a linear combination of the right-handed and left-handed configurations.
There are two such linear combinations. If these two combinations are chosen to be
of equal amplitude and orthogonal to each other, then one will be a symmetric
representation (the state) and the other will be an antisymmetric representation
(the state). These two states behave differently when subjected to a gradient electric
field.
In the ammonia maser at Columbia University, molecules were introduced into a
high-vacuum chamber and then collimated into a beam. The beam of molecules then
traversed an electric field that spatially separated the molecules of the two states. Molecules in the lower-energy state were rejected, while molecules in the excited state were
sent to the exit beam. These excited molecules were permitted to drift into a region of
stimulating electromagnetic radiation, where amplification took place.
Shortly afterward, the Central Radio Propagation Laboratory of the National Bureau of Standards used the ammonia maser amplifier, a self-sustained oscillator, as a
standard clock. Its power output was of the order of 1 microwatt, and its stability was
estimated at a few parts in 1010, an order of magnitude better than that of the crystalcontrolled oscillator locked to the ammonia (3, 3) inversion line. The improvement
was due to the maser’s much narrower line width, since collision broadening of the
energy levels is virtually eliminated in the unidirectional, focused molecular beam.
Furthermore, Doppler broadening of energy levels is greatly reduced through the use
of molecular velocity selection.
Following the development of the ammonia maser, G. Makov, C. Kikuchi, J. Lambe,
and R. W. Terhune reported another device.10 This was the solid-state ruby maser,
which used three of the chromium ion’s four electronic spin levels of its ground electronic state. Ruby is the mineralogical name for crystalline corundum (Al 2 O 3 ) having
trivalent chromium ions as principal impurities. The maser achieved continuous
electron-spin population inversion by applying a strategy — suggested by Bloembergen
and similar to that known as optical pumping — that achieves preferential population
8N.
G. Basov and A. M. Prokhorov, Zh. Eksp. Theo. Fiz. 27:431, 1954.
P. Gordon, H. J. Zeiger, and C. H. Townes, Phys. Re v. 99:1264, 1955.
10G. Makov, C. Kikuchi, J. Lambe, and R. Terhune, Phys. Rev. 109:1399, 1958.
9J.
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of excited optical states.11 Since the electronic spin states’ energy levels of the trivalent
chromium ion in ruby depend on the strength of an applied magnetic field, the ruby
maser proved to be tunable. It quickly gained use as a high-gain, low-noise amplifier, a
boon to radio astronomy. The ruby crystal used by Kikuchi and his associates as the active element in their maser was donated by the University of Michigan to the Smithsonian Institution in Washington, D.C., and today resides there as a permanent exhibit.
Another important milestone in maser history came in 1960 when H. M. Goldenberg,
D. Kleppner, and N. F. Ramsey reported on the development and operation of a hydrogen maser having the astonishing stability of 1 part in 30 1012 at a frequency of
1 420 405 751.786 Hz.12 Since there are about 31 million seconds in a year, this maser
will neither gain nor lose more than a few seconds in 100 000 years. The
Michelson–Morley experiment was repeated to ever-increasing accuracy with this very
precise clock, thereby verifying the conclusion of the original experiment — that the
speed of light when measured in an inertial reference frame is invariant and is a universal constant.
FROM MASERS TO LASERS
Almost immediately upon the development of the ammonia maser, A. L. Schawlow and
C. H. Townes suggested that the concept be extended to optical wavelengths.13 The
acronym laser, by analogy with maser, was coined to stand for light amplification by stimulated emission of radiation. The rush to invent was on, and the first laser to be reported was T. H. Maiman’s ruby laser at the Hughes Corporation Research Laboratories.14 The ruby laser consisted of a single crystal rod of synthetic corundum with
chromium ions substituted for aluminum ions to the extent of a few parts per thousand. The ends of the rod were polished to be planar and parallel to each other and
then metallized to form a radiation cavity resonator. The rod was surrounded by a helical flash lamp (Fig. 1). When a bank of capacitors in the flash lamp discharged, they
caused the ruby rod to emit a single pulse of coherent and monochromatic radiation
of wavelength 694.3 nm. The pulse duration was of the order of 108 seconds, and the
laser had a power output of some 107 W/pulse. Due perhaps to media publicity, laser
became a household word almost overnight, and the verb to lase was coined to stand for
the action of a laser.
The laser has evolved into an indispensable tool in both research and engineering.
Its numbers, varieties, and applications have experienced astonishing growth. Lasers
are used in medicine as precision scalpels. Researchers have focused intense
laser beams on targets in attempts to initiate controlled thermonuclear fusion (see
Chapter 14). Scientists bounce laser light off of total internal reflectors on the Moon’s
surface to measure moonquakes. Lasers are used for precision machining and for such
mundane tasks as reading bar codes on railroad rolling stock and on grocery-store
items.
Nature has produced her own masers. Radio astronomers have observed interesting
radiations emanating from extraterrestrial space, believed to be associated with interstellar H2O molecules, electrically neutral OH and SiO free radicals, and perhaps other
species. On the basis of the directions in space from which these radiations emanate,
their narrow spectral bandwidths, and their radiation intensities, physicists have deduced that the radiations originate in naturally occurring masers. Although some
11N.
Bloembergen, Phys. Rev. 104:324, 1956.
M. Goldenberg, D. Kleppner, and N. F. Ramsey, Phys. Re v. Lett. 5:361, 1960; Phys. Rev.
126:603, 1962.
13A. L. Schawlow and C. H. Townes, Phys. Rev. 112:1940, 1958.
14T. H. Maiman, Nature 187:493, 1960.
12H.
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Laser beam
Metal case
Partially
reflecting
surface
Flash lamp
Ruby rod
Totally
reflecting
surface
(a)
(b)
Figure 12.1.1 (a) A flash lamp surrounds a ruby rod. Chromium ions in the rod absorb light from the flash lamp, which pumps electrons to an excited state. A spontaneous emission from the excited state stimulates other ions to emit. The stimulated
emission at 694.3 nm bounces back and forth between the reflecting ends of the rod,
building up a coherent laser beam. (Adapted from D. Ebbing, General Chemistry, 5th
ed., Boston, Houghton Mifflin Co., 1996) (b) A photograph of the first ruby laser.
(Courtesy of HRL Laboratories, LLC)
maser-burst radiations are known to issue from the Sun, many cosmic masers are most
likely either galactic or extragalactic in origin.15,16
Acknowledgments The authors would like to express indebtedness to Prof. J. Weber
of the University of Maryland for his critical reading of the manuscript and to Mary
Beth Riedner of Roosevelt University for providing valuable assistance.
Suggestions for Further Reading
N. F. Ramsey, Science 248:1612, 1990. A review of the hydrogen atom maser.
15M.
16M.
Miyoshi, Nature 371:395, 1994.
Elitzur, Scientific American 272:78, 1995.
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